This invention relates to a scrambling and descrambling system for a CATV broadcasting system operable for protecting the CATV system from unauthorized reception of programs.
In a CATV broadcasting system, pay programs and noncharged programs are broadcast simultaneously. For the purpose of preventing reception of pay programs by unauthorized subscribers, the visual signals of the pay programs are altered at the transmitter by a scrambling device, and demodulated at the receiving terminal by a descrambling device.
For the realization of such a procedure, a gray sync method or the like is well known. According to this method, not only the visual signal is scrambled as described above, but also the aural signal is scramble modulated as a key signal. Although such a procedure is advantageous in that the descrambling can be carried out by comparatively simple circuitry, it is accompanied by the following problems:
A. In a terminal device, feedback circuits are used for reducing the envelope of the scrambling wave to zero. However, in order to reduce this wave to zero, a high loop gain with a high error sensitivity is required, so that the circuit tends to become unstable.
B. To eliminate this difficulty, there is a method wherein the degree of modulation is differentiated between the visual signal and the aural signal, so that when the visual signal is completely demodulated, a key signal still remains in the aural signal. However, according to this method, since the remaining signal has been preset at the scrambler, the relation between the modulations of the aural signal and the visual signal will differ from 100% coincidence. More specifically, the image quality at the demodulating side is deteriorated by the preset deviation of the aural signal portion. In addition, because detection at the demodulating side for the realization of reduction to zero of the output envelope wave is carried out by detecting a predetermined amount of the remaining key signal, the method is susceptible to the variation in characteristics of the detector and the loop circuit, thus revealing the disadvantage of instability.